1. Field of the Invention
The present invention generally relates to a constant voltage generating circuit, and more particularly relates to a band-gap type constant voltage generating circuit produced in a semiconductor chip.
2. Description of the Related Art
Such a band-gap type constant voltage generating circuit is constituted such that a constant voltage is output by utilizing a difference of a potential drop (which is defined as a base-emitter voltage) in a P-N junction between a base and an emitter of a bipolar junction transistor. This band-gap type constant voltage generating circuit can output the constant voltage without being substantially influenced by variation in environmental temperature.
Nevertheless, the band-gap type constant voltage generating circuit is susceptible to variation in process conditions. When a plurality of semiconductor chips, each of which has a band-gap type constant voltage generating circuit, are processed and produced in a semiconductor wafer, such as a silicon wafer, various elements forming the band-gap type constant generating circuit are subjected to both an absolute process fluctuation and a relative process fluctuation due to the variation in the process conditions.
For example, when two elements, which are identical to each other, are processed and produced in a silicon wafer at locations remotely separated from each other, a variation between the produced elements is defined as the absolute process fluctuation. On the other hand, when two elements, which are identical to each other, are processed and produced in a silicon wafer at locations closed to each other, a variation between the produced elements is defined as the relative process fluctuation. In general, the absolute process fluctuation is on the order of ±20%, and the relative process fluctuation is on the order of ±2%.
Of course, the absolute process fluctuation should be eliminated before quality and reliance of the band-gap type constant voltage generating circuits can be improved. Nevertheless, the prior art band-gap type constant voltage generating circuits fail to eliminate the absolute process fluctuation, as discussed in detail hereinafter.